1. Field of the Invention
The present invention relates to a rack driven electric power steering apparatus, and more particularly to a rack driven electric power steering apparatus which absorbs an impact and vibrations transferred from a road surface to a wheel and a rack bar, absorbs noise generated in the rack bar and a rack housing when the rack bar is slid, making an operation of the electric power steering apparatus smooth, reducing noise, and providing a driver with a comfortable steering feeling.
2. Description of the Prior Art
Although hydraulic power steering apparatuses using a hydraulic pressure of a hydraulic pump have been widely used as power steering apparatuses for a vehicle, electric power steering apparatuses using an electric motor are gradually becoming common recently.
A general electric power steering apparatus includes a steering system extending from a steering wheel to two wheels, and an auxiliary power mechanism for supplying an auxiliary steering power to the steering system.
The auxiliary power mechanism includes a torque sensor configured to detect a steering torque applied to the steering wheel by a driver and output an electric signal proportional to the detected steering torque, an electronic control unit (ECU) configured to generate a control signal based on an electric signal transferred from the torque sensor, a motor configured to generate an auxiliary steering power based on a control signal transferred from the electronic control unit, and a belt type transmission unit configured to transfer an auxiliary power generated in the motor to a rack bar by means of a belt.
FIG. 1 is a partially sectional view schematically illustrating a conventional rack driven electric power steering apparatus, and FIG. 2 is a sectional view schematically illustrating a part of the conventional rack driven electric power steering apparatus.
As illustrated in FIGS. 1 and 2, the conventional rack driven electric power steering apparatus includes a rack bar 140 extending in a transverse direction of a vehicle and including a rack gear on an outer peripheral surface thereof, a pinion shaft 110 including a pinion gear engaged with the rack gear, a ball nut 210 engaged with an outer screw groove 145 by means of balls, a belt type transmission unit 150 configured to connect the ball nut 210 and a motor shaft 155, and a motor 150.
The pinion shaft 110 is connected to the steering wheel through a steering shaft, and the rack bar 140 having the outer screw groove 145 having a predetermined length on an outer surface thereof is installed within a rack housing 120.
The belt type transmission unit 160 includes a belt connecting the motor shaft 155 and the ball nut 210, and transfers an auxiliary steering power generated in the motor 150 in proportion to a steering torque applied to the steering wheel to the rack bar 140 through the ball nut 210.
The conventional rack driven electric power steering apparatus of FIG. 2 includes a rack bar 140 having an outer screw groove 145 on an outer peripheral surface thereof, a ball nut 210 having an intermediate path 215 along which balls 220 circulate, balls 220 configured to roll while contacting the outer screw groove 145 of the rack bar 140 and the intermediate path 215, and an end cap 230 attached to an end of the ball nut 210.
The rack driven electric power steering apparatus slides the rack bar 140 while the balls 220 roll while contacting the outer screw groove 145 of the rack bar 140 and an inner screw groove of the ball nut 210.
However, according to the conventional rack driven electric power steering apparatus, the end cap is exposed to the outside of the ball nut. Further, impact noise and friction noise increase while the balls are circulating because directions of the balls are converted rapidly. Furthermore, the end cap should be assembled again after the balls are introduced using an assembling device, making the process inefficient.